1. Field of the Invention
The invention relates to the manufacture of thermoelectric elements (also referred to hereinafter as “TE elements”) as well as to the products of the method and to the use of those products.
2. Background
The thermoelectric effect (also referred to hereinafter as “TE effect”) was already discovered and described by Peltier and Seebeck in the 19th century. It was found that a relationship exists between the heat currents and electrical currents flowing through combinations of different metals, alloys or semiconductors (also referred to hereinafter as “TE materials”). On the one hand, a heat flow can create an electrical potential between the hotter and colder end of the TE material, and this can be exploited in the form of a current flow through a closed electrical circuit (Seebeck effect, thermoelectric generator). On the other hand, the application of an electrical potential to such materials leads not only to a current flow but also to a heat flow; i.e. one electrical contact face becomes hotter and the other becomes cooler (Peltier effect, Peltier elements).
Thermoelectric devices are also known as heat sensors for temperature measurement. Furthermore, they can also be used as heat pumps for cooling purpose. A detailed discussion of the scientific research and of the prior art can be found in “Thermoelectrics handbook: macro to nano”, D. M. Rowe, CRC Press, 2006.
The TE effect has already been exploited in numerous applications for many years.
For example, thermoelements are used for temperature measurement. Peltier elements heat/cool in applications with low power demand or where compression refrigerating systems cannot be used for other reasons. Also known are thermoelectric generators (TEG), which generate electrical current from existing heat currents.
Thermoelements and Peltier elements are already available as mass-produced products. TEGs are on the threshold of becoming mass-produced products. For broad application, however, their unit costs relative to electrical output power are still too high and the efficiency is too low, especially in the case of TEGs.
In the present description, thermoelements, Peltier elements and thermoelectric generators are also designated as thermoelectric elements (also referred to hereinafter as “TE elements”).
The quantity known as thermoelectric potential, or in other words the electrical voltage that can be generated, depends on the material-specific properties of the TE material, on the Seebeck coefficient and on the temperature difference. High Seebeck coefficients and large temperature differences lead to high thermoelectric voltages. In order to be able to draw large electrical power, a large heat flow must pass through a material with very good thermal insulation capacity. This necessarily means that large areas must be provided or/and very high Seebeck coefficients should exist. With the present methods for manufacturing TE elements, either it is not possible in practice to manufacture a large-area element or the manufacture of such TE elements would lead to exorbitant manufacturing costs. Research and development of the past years has therefore concentrated predominantly on increasing the Seebeck coefficient with new TE materials.
In recent years, the developments in nanotechnology have been accompanied by a noteworthy improvement in efficiency of TE materials. Via thin-film coatings or nanotube techniques, it is possible to produce two-dimensional or one-dimensional thermoelectric structures having a better TE effect compared to traditional “bulk” materials. Manufacturing methods based on these thin-film coatings also do not solve the problem of manufacturing large-area, inexpensive elements. As regards applications, these methods concentrate on the manufacture of microcomponents, such as chip coolers as well as current generators for microprocessors or wrist watches. Examples of known manufacturing methods can be found in DE 102 32 445 A1.
Thermoelectric generators and Peltier elements have therefore found broad application in areas in which they can be used as microcomponents. And in niches in which manufacturing costs are not decisive, such as space flight and satellite technology, thermoelectric elements have already been used successfully for decades. Nuclear reactors, for example, are used as the heat source.
In addition, however, numerous potential applications for TE elements exist. Because of the ever-shrinking energy resources and simultaneously rising energy demand, the use of thermoelectric generators as a renewable energy source would be of particular interest. There can be found numerous unexploited heat sources from which at least some electrical current could be obtained. Examples include:                Hot exhaust gases and wastewaters being discharged into a cooler environment.        Areas heated by the sun or other sources.        Floors and mats intended to insulate a cooler substructure.        Process technology functions that operate with large temperature differences, such as evaporation of cryogenic media (e.g. liquefied natural gas).        exploitation of the motor-vehicle exhaust gas heat as an electricity generator (fuel savings).        
The use of TE elements as generators in the motor-vehicle exhaust pipe has already been tested in practice. In this case also, the manufacturing costs of the systems still argue against introduction into the automobile market (in this regard see “Commercialization of Thermoelectric Technology”, Francis R. Stabler, General Motors Corporation. Mater. Res. Soc. Symp. Proc. Vol. 886, 2006 Materials Research Society).
Besides the difficulty of manufacture of large areas, all the extremely diverse TE elements that have already been applied and the potential areas of application imply yet another problem. The different areas of application involve very different material requirements. In some cases the temperatures are very high, above 1000° C. (e.g. nuclear reactor as the heat source); in other cases the temperature levels are very low (e.g. cryogenic evaporators). In some cases large temperature differences can be exploited; in other cases the material composition must be optimized for small available temperature differences. In some cases rigid structures of the TE elements are sufficient; in other cases it would be advantageous to have flexible TE elements. Furthermore, the external geometry, length, width and thickness of the TE elements should be adaptable as flexibly as possible to the application situation. The consequences are extremely diverse manufacturing methods, which sometimes are unique for individual applications. This circumstance makes the market for TE elements very highly segmented, and from the viewpoint of business economics makes it much more difficult to begin with developing production methods.
Intensive research and development work has led in recent years to the development of progressively more powerful TE materials. Nevertheless, the major part of research and development is concentrated on the development of new materials, whereas questions related to manufacturing methods and application techniques have commanded less attention.
The conventional process for manufacturing TE elements typically consists of the following steps:                manufacture of TE materials doped in different ways (e.g. batchwise in shaking furnaces),        airtight fusing of the metal mixtures in glass ampoules,        crystal growth by vertical zone melting in the glass ampoules,        sawing of the metal rods obtained in this way into slices (“wafers”) of a few millimeters thickness,        sputtering the wafer surfaces with contact enhancers (e.g. nickel),        sawing of the wafers into cuboids (“legs”),        arrangement of the n-legs and p-legs alternately in masks (matrices),        placing of contact plates along with electrical contact zones and leads on both sides of the leg matrices,        sintering of the obtained sandwich to the finished composite, and        application of external electrical insulating layers.        
The first four steps are typical semiconductor processing steps with high cleanliness requirements and little potential for automation. Also obvious is the large number of necessary piece-by-piece manipulations on the most diverse workpieces in the subsequent steps. Here also it is difficult to implement automation and continuous manufacture.
Already known manufacturing methods for TE elements are largely oriented toward methods that have become familiar from the manufacture and processing of semiconductors.
As an example, a method for manufacturing a thermoelectric layer structure is described in DE 102 30 080 A1. The manufacturing method is based on traditional Si wafer technology, wherein the different functional layers are applied successively to the wafer and structured by etch processes.
In DE 102 31 445 A1 a continuous manufacturing method for TE devices is described. Therein the alternating structures of p-doped and n-doped TE semiconductors typical of TE components are produced as continuous areas on insulating plastic films, which are then laminated on top of each other by winding them in a plurality of layers onto a drum. Pieces/strips are then cut out and provided with electrical contacts at the face sides, in order to obtain the necessary series interconnection of numerous alternating n-legs and p-legs. With this method it is not possible to produce flat arrays of TE legs; it is possible only to produce strips, through which electric current flows lengthwise. In order to obtain flat structures, either broad strips must be cut (in which case very high electric currents flow through them), or a flat structure must be pieced together from a plurality of strips.
From U.S. Pat. No. 6,300,150 B1 there is known a method in which thin-film TE devices are produced on wafers by means of traditional semiconductor technology. The manufacture of usable TE modules is achieved by traditional cutting of the raw wafer and reassembly in the needed n-p arrangement.
U.S. Pat. No. 6,396,191 B1 describes the construction of TE components composed of numerous TE-active layers with intermediate layers along the heat flow. Depending on the local temperature level within the layer structure suitable TE materials are used. Thus the concept of functionally graded TE elements is implemented here, in order to achieve the highest possible duty factors and usable electrical voltages by this interconnection. The manufacturing methods described in this patent fall within the range of traditional semiconductor processing and coating technologies.
With few exceptions, therefore, TE devices that have been available heretofore have not yet gained a foothold in the mass market, despite a principally achieved technical maturity and numerous demonstration systems. This has several reasons:                High specific costs resulting from the manufacturing process (large number of usually expensive process steps).        High consumption of expensive TE material.        High specific weight of the modules.        Low flexibility in shaping the modules.        